The present invention relates to wireless communications links and specifically to high data rate point-to-point links.
Wireless communications links, using portions of the electromagnetic spectrum, are well known. Most such wireless communication at least in terms of data transmitted is one way, point to multi-point, which includes commercial radio and television. However there are many examples of point-to-point wireless communication. Mobile telephone systems that have recently become very popular are examples of low-data-rate, point-to-point communication. Microwave transmitters on telephone system trunk lines are another example of prior art, point-to-point wireless communication at much higher data rates. The prior art includes a few examples of point-to-point laser communication at infrared and visible wavelengths.
The need for faster (i, e., higher volume per unit time) information transmission is growing rapidly. Today and into the foreseeable future transmission of information is and will be digital with volume measured in bits per second. To transmit a typical telephone conversation digitally utilizes 5,000 bits per second (5 Kbits per second). Typical personal computer modems connected to the Internet operate at, for example, 56 Kbits per second. Music can be transmitted point to point in real time with good quality using mp3 technology at digital data rates of 64 Kbits per second. Video can be transmitted in real time at data rates of about 5 Mbits per second. Broadcast quality video is typically at 45 or 90 Mbps. Companies (such as telephone and cable companies) providing point-to-point communication services build trunk lines to serve as parts of communication links for their point-to-point customers. These trunk lines typically carry hundreds or thousands of messages simultaneously using multiplexing techniques. Thus, high volume trunk lines must be able to transmit in the gigabit range (billions of bits per second). Most modem trunk lines utilize fiber optic lines. A typical fiber optic line can carry about 2 to 10 Gbits per second and many separate fibers can be included in a trunk line so that fiber optic trunk lines can be designed and constructed to carry any volume of information desired virtually without limit. However, the construction of fiber optic trunk lines is expensive (sometimes very expensive) and the design and the construction of these lines can often take many months especially if the route is over private property or produces environmental controversy. Often the expected revenue from the potential users of a particular trunk line under consideration does not justify the cost of the fiber optic trunk line. Digital microwave communication has been available since the mid-1970""s. Service in the 18-23 GHz radio spectrum is called xe2x80x9cshort-haul microwavexe2x80x9d providing point-to-point service operating between 2 and 7 miles and supporting between four to eight T1 links (each at 1.544 Mbps).
Bandwidth-efficient modulation schemes allow, as a general rule, transmission of data at rates of 1 to 10 bits per Hz of available bandwidth in spectral ranges including radio wave lengths to microwave wavelengths. Data transmission requirements of 1 to tens of Gbps thus require hundreds of MHz of available bandwidth for transmission. Equitable sharing of the frequency spectrum between radio, television, telephone, emergency services, military and other services typically limits specific frequency band allocations to about 10% fractional bandwidth (i.e., range of frequencies equal to about 10% of center frequency). AM radio, at almost 100% fractional bandwidth (550 to 1650 GHz) is an anomaly; FM radio, at 20% fractional bandwidth, is also atypical compared to more recent frequency allocations, which rarely exceed 10% fractional bandwidth.
Reliability typically required for wireless data transmission is very high, consistent with that required for hardwired links including fiber optics. Typical requirements are error rates of less than one bit in ten billion (10xe2x88x9210 bit-error rates), and link availability of 99.999% (5 minutes of down time per year). This necessitates all-weather link operability, in fog and snow, and at rain rates up to 100 mm/hour in many areas.
In conjunction with these availability requirements, weather-related attenuation limits the useful range of wireless data transmission at all wavelengths shorter than the very long radio waves. Typical ranges in a heavy rainstorm for optical links (i.e., laser communication links) are 100 meters and for microwave links, 10,000 meters.
Atmospheric attenuation of electromagnetic radiation increases generally with frequency in the microwave and millimeter-wave bands. However, excitation of rotational transitions in oxygen and water vapor molecules absorbs radiation preferentially in bands near 60 and 118 GHz (oxygen) and near 23 and 183 GHz (water vapor). Rain, which attenuates through large-angle scattering, increases monotonically with frequency from 3 to nearly 200 GHz. At the higher, millimeter-wave frequencies, (i.e., 30 GHz to 300 GHz corresponding to wavelengths of 1.0 millimeter to 1.0 centimeter) where available bandwidth is highest, rain attenuation in very bad weather limits reliable wireless link performance to distances of 1 mile or less. At microwave frequencies near and below 10 GHz, link distances to 10 miles can be achieved even in heavy rain with high reliability, but the available bandwidth is much lower.
What is needed is a wireless data link that can provide data rates in excess of 1 Gbps over distances up to ten miles in all weather conditions except the most severe, and which link will not go down or deliver corrupted data even in the most severe weather conditions.
The present invention provides a point-to-point, wireless, millimeter wave communications link providing data transmission rates of over 1 billion bits per second (more than 1 Gbps) at ranges of several miles during normal weather conditions. In a preferred embodiment a communication link operates within the 92 to 95 GHz portion of the millimeter spectrum. A first transceiver transmits at a first bandwidth and receives at a second bandwidth both within the above spectral range. A second transceiver transmits at the second bandwidth and receives at the first bandwidth. The transceivers are equipped with antennas providing divergence of less than 10xe2x88x924 steradians so that an almost unlimited number of transceivers can use the same spectrum. In a preferred embodiment the first and second spectral ranges are 92.3-93.2 GHz and 94.1-95.0 GHz. During very bad weather conditions this data link will not provide a 10xe2x88x9210 bit-error rate and 99.999% availability. Therefore, a second transceiver pair operating in the 3 to 30 GHz microwave range acts as a backup to achieve the above bit-error rate and availability requirements, but at a temporarily lower data rate. In this embodiment a network management system, interfacing the link with the network switching and routing hardware, continually senses the performance of the high data rate channel and switches to the low data rate channel when the performance of the high data rate channel falls below a set threshold.